In order to overcome the double fading effect in RIS-assisted systems, some proposals have been raised and attracted a lot of interest.
In [1]}, the authors proposed a novel relay-aided RIS architecture consisting of two RISs connected via a full-duplex relay. This architecture can achieve the same performance with conventional RIS while only requiring much fewer reconfigurable elements. However, they only considered the single-input single-output (SISO) system and derived the theoretical upper bound of the achievable rate.
The designs of BS transmit beamforming and RIS reflection have not been well investigated. A further question is that the first RIS in their proposed system only reflects the signal towards relay and cannot simultaneously serve the users around it. In [2]}, the authors proposed a semi-active RIS-aided architecture, in which the RIS not only passively reflects the signal, but also actively amplifies it at the same time. However, the practical implementation of this hybrid-RIS remains an open problem, while it needs expensive RF chains.
